json.parse

def create
@contract = Contract.new(contract_params)

    require 'json'
    @contract.save!
    JSON.parse(params[:whitelist]).each do |address|
      NftOwner.create auth_type: 'address', auth_value: address, contract_id: @contract.id, manager: current_manager.email
    end
    redirect_to @contract, notice: '操作成功'
end

使用双引号在placeholder，需要进行外层的单引号进行调用内层的双引号